yn is the one headstamp of Israel Miliary Industries. Note the first two big letters on the box and the headstamp on the cartridge. (I've no idea how to make a backwards y and upsidedown n to simulate the Hebrew letter.)
It's 7.62mm NATO. The boat tailed bullet may, or may not make it more accurate. US 7.62mm NATO is normally blunt tail.
Let us know how it shoots, it's 30 years old.
-- Chuck

I think it was 1978 where IMI switched to Roman characters instead of Hebrew on their headstamps. I have a bunch of both, and it was either '77 or '78 where they switched. After that, the headstamp for their military ammo is TZ or TZZ. "IMI" is only used on commercial ammo.
-Troy